Prop shops who sell and repair props around here will allow you to test before you buy "IF" you return the test prop you didn't want in the same condition it was in when you picked it up. In short, go to known clean water, make your test and get it off. If you want to stay around and boat some more, take along your old prop and put it on. You will pay more initially for the prop but it will be the cheapest in the long run and the one with which you will be satisfied..........and just what's that worth?
Mark